The coating of preferably band-like substrates with a barrier layer is an important processing step in the manufacture of various packaging materials. In particular polymer materials, after vapour-deposition of a thin metal layer (e.g. aluminium), are further processed to form packaging materials which exhibit a high barrier effect against oxygen and water vapour, possibly also against aromas.
Packaging materials containing a thin metal layer are opaque and exhibit a high microwave absorption, which is disadvantageous for some uses in the field of foodstuff packaging. This is why metallic barrier layers are increasingly being replaced by various oxide-based barrier layers (oxide of Si, Al, Mg).
The reactive evaporation of aluminium from the boat evaporator offers the possibility of combining the advantage of a low evaporation temperature with high optical transparency and a good microwave permeability of the finished product. Methods using electron beam or induction evaporators can also be used.
In barrier coatings of this kind, the optical transmission depends on the mixing ratio of aluminium and its oxide—hereinafter also referred to as stoichiometry. As the oxide content increases, the optical transmission rises, while the barrier effect of the layer is reduced. There is a working area in the slightly hypostoichiometric range whose lower limit is determined by the maximum allowed absorption and whose upper limit is determined by the required minimum values of the barrier [Schiller, N.; Reschke, J.; Goedicke, K.; Neumann, M.: Surface and Coatings Technology, 86-87 (1996) 776-782].
Therefore, it is common practice to make sure that this working area is observed by providing the reactants aluminium and oxygen in a certain adjustable ratio of ingredients (JP 62-103 359 A).
Since the visual evaluation of the finished product plays an important role, especially in the mass packaging field, and minor variations of transmission can be readily recognised, numerous attempts have been made to achieve a uniform transmission across large coating areas; as well as the uniform transmission, in order to achieve a barrier effect one must always strive for a sufficient adhesion of the layer.
It is known that by measuring the optical transmission of the reactively vapour-deposited layers, the process can be controlled by adjusting the evaporation rate such that, with a pre-specified gas flow, a predetermined transmission value is maintained. This is realised in the case of reactive electron beam evaporation (DE 44 27 581 A1). This form of control is not feasible using a boat evaporator, since the evaporator boats are unsuitable for rapid alterations of the heating current and thus for the evaporation rate, and too inert for such a control.
It is further known that by adjusting a certain reactive gas flow that at constant evaporation rates results in weakly absorbing layers, the mixing ratio of the evaporated metal and its oxide can be kept substantially uniform (EP 0 437 946 B1). There is, however, the disadvantage that the process is technologically complex, as—especially when using several evaporator boats—there are high requirements in terms of keeping the evaporation rate constant.
It is further known that aluminium oxide layers can be vapour-deposited in a clearly hypostoichiometric amount. At first, the optical transmission obtained is below the above-mentioned working area. In a secondary oxidation step that is carried out directly after the vapour-deposition (and is plasma-activated) or without any activation during an additional winding operation, the layer is additionally brightened (EP 0 555 518 B1, EP 0 695 815 B1). This additional processing step, however, requires a higher degree of technology.
It is further known that the secondary oxidation (that may be plasma-activated) in the case of a vapour-deposition of very thin, hypostoichiometric oxide layers and a subsequent, relatively long dwelling time in the reactive gas atmosphere spontaneously results in a sufficient brightening of the layer. By using a tape drive which considerably extends the path of the vapour-deposited substrate in the recipient via additional reversing rolls and also allows it to undergo the vapour-deposition cycle several times, sufficiently transparent layers of the usual thickness can be vapour-deposited by superimposing several very thin layers, each subjected to secondary oxidation, by vapour-deposition (U.S. Pat. No. 5,462,602). The method, however, requires a considerably increased extent of mechanical performance at the tape drive.
It is known that barrier layers that, individually, do not meet the minimal requirements concerning their barrier effect, show sufficient barrier effects when used in a multiple layer system. The combination of dusted and vapour-deposited layers is a solution that has also been proposed for aluminium oxide (DE 43 43 040 C1). In this case, the coating is carried out at very different deposition rates. However, the implementation of several successive processing steps causing different processing times also requires a considerably higher degree of technology. In addition, the barrier effect of such layers is often restricted by the fact that different layer stresses are generated in the individual layers; proceeding from the transition areas this can gradually lead to the formation of cracks.
It is further known that good barrier properties can be obtained by implementing the process such that the density of the barrier layer is prevented from falling below a certain limiting value. Therefore, 2.7 g/cm3 is claimed as the lower limiting value of density for aluminium oxide (EP 0 812 779 A2). However, since the density of a vapour-deposition layer strongly depends on the condensation conditions, a preset minimum value is always a considerable restriction in the process implementation. In particular, when retrofitting existing aluminium vapour-deposition facilities for the process of reactively vapour-depositing aluminium oxide layers, the constructive circumstances often determine the condensation conditions, whereby alteration is very costly and can make the conversion as a whole uneconomical. In addition to this, the density of the layer is not a parameter that can be directly measured during the process.
It is further known that the substrate can be provided with a thin nucleation layer having an approximate thickness of 5 nm prior to coating with the actual barrier layer (U.S. Pat. No. 5,792,550). In many cases, this also requires an additional processing step which considerably increases the production costs.
It is further known that the substrate can be treated with a magnetron plasma prior to coating it with the barrier layer [Löbig, G. et al.; SVC 41st Annual Technical Conference Proceedings (1998) S.502]. This gives activates and cleans the substrate surface and improves the adhesion of the layer, thus making the barrier effect less dependent on the stoichiometry of the barrier layer. For many applications, however, the barrier effect obtained in this way is not sufficient either.
Finally it is known that the barrier layer can be vapour-deposited with a stoichiometric gradient (DE 198 45 268 C1). However, the exact adjustment of the stoichiometry in the interface between the layer and the substrate is a very sensitive parameter in such layers.